A belt driving device that includes an endless belt is used in an electrographic type image forming apparatus to carry a recording medium.
The belt driving device bridges a belt on a two parallel rollers and drives the belt by rotating one of the rollers. However, while driving, the belt may skew due to the parallelism of the two rollers, uneven tension of the belt, dimensional accuracy of each roller and the like.
Therefore, a belt driving device has been proposed that suppresses the skew of the belt by changing inclination of one of the rollers in response to the skew state of the belt (see Japanese Laid-Open Patent Application No. 2006-162659 (claim 1, paragraphs 0038-0043 and FIGS. 11-13). In this belt driving device, a pulley that guides side edge parts (edge parts in the width direction) of the belt is provided movably in the axial direction of the roller, and a roller shaft displacement member is provided that displaces one end of a roller shaft up and down in response to the movement of the pulley.
However, in the conventional belt driving device, a large load (applied force, frictional force, etc.) is applied to the side edge part of the belt as the side edge part of the skew belt contacts the pulley. Conventionally, life of the belt is determined by assuming that the belt driving device is used under such conditions. However, recently, further extension of the life of the belt is desired.
The present invention considers the above-described problem and has an object to suppress the load to the belt and to extend the life of the belt and the belt driving device.